如果没有记录，则设置事件以删除表

Question

I am trying to create a table bus with 2 events. One AutoDelete to maintain the data in the bus table (This one works) and the other one to delete the table if no records exist in the table. AutoDelete I dont know how can I set the condition as SELECT COUNT(*) FROM bus in the event DropBusTable to get it to work since I dont have time as the other one I have condition.

I appreciate any help.

   stt.execute("CREATE TABLE IF NOT EXISTS bus"
            + "(id INT(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,"
            + "mac VARCHAR(30) NOT NULL UNIQUE,"
            + "route int(11) NOT NULL,"
            + "latitude FLOAT(10,6) NOT NULL,"
            + "longitude FLOAT(10,6) NOT NULL,"
            + "created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP)");

    stt.execute("CREATE EVENT IF NOT EXISTS  AutoDelete "
            + "ON SCHEDULE EVERY 3 MINUTE "
            + "DO "
            + "DELETE FROM bus WHERE created_at < (NOW() - INTERVAL 3 MINUTE)");

    // I tried this statement here but it does not work.
    stt.execute("CREATE EVENT IF NOT EXISTS  DropBusTable "
            + "Do "
            + "DROP TABLE IF EXISTS  bus");

Answer 1

You can reset the counter with:

ALTER TABLE <table_name> AUTO_INCREMENT = 1

by adding it to your existing task:

  stt.execute("CREATE EVENT IF NOT EXISTS  AutoDelete "
            + "ON SCHEDULE EVERY 3 MINUTE "
            + "DO BEGIN"
            + "DELETE FROM bus WHERE created_at < (NOW() - INTERVAL 3  MINUTE);"
            + "ALTER TABLE bus AUTO_INCREMENT = 1 ;"
            + "END;");

See this article for more info.